The computer hackers, chat room denizens and young people who comprise the loosely affiliated Internet collective have increasingly turned to questionable tactics, drawing the attention of the FBI, the Department of Homeland Security and other federal investigators.

What was once a small group of pranksters has become a potential national security threat, federal officials say.

The FBI has carried out more than 75 raids and arrested 16 people this year in connection with illegal hacking jobs claimed by Anonymous.

Some members of the group have called for shutting down Facebook in November over privacy issues, although other Anonymous followers are disavowing such an attack — underscoring just how loosely organized the group is and how problematic it is to police.

SAN FRANCISCO Anonymous is not so anonymous anymore.The computer hackers, chat room denizens and young people who comprise the loosely affiliated Internet collective have increasingly turned to questionable tactics, drawing the attention of the FBI, the Department of...